Spectrophotometric Isolation of Kinetically Different Pools of P-700 and Their Correlation to the Reduction of NADP by Isolated Chloroplasts *. I. The Effect of Light Quality and Intensity

Excitation of isolated chloroplasts in the presence of ferredoxin and NADP by repeated short flashes yields a polyphasic absorption change at 700 nm. Assuming first-order reactions, the signal may be resolved into three distinct components with average relaxation times of approxi­mately 20 μs, 150 μs and 20 ms. Their relative magnitude is dependent on experimental conditions; their spectral characteristics indicate that all three components may be ascribed to P-700.

Concurrent measurements of Y-NADPH, the flash yield of NADP reduction with an enzymatic recycling method, allowed Y-NADPH to be compared to the magnitude of each of the three P-700 components and to total P-700. In general, the data show a good correlation of NADP reduction with the sum of the μs-phases but not with the ms-phase or total P-700.

Analysis of light intensity curves (blue or far red flashes) with a mathematical model which yields maximum values for all parameters at infinite light intensity shows that in both cases approximately two moles of the microsecond component of P-700 turn over for each mole of NADPH formed. In contrast, the molar ratio of the ms-component to the yield of NADP reduction is approx. 0.2 in blue and approx. 6.3 in far red light. The data suggest that only that portion of the P-700 pool which relaxes in the microsecond range may be involved in the reduction of NADP while the ms-component is funtionally isolated from linear electron transport.